In Wireless Communications networks, for example, in Third Generation Partnership Project Long Term Evolution Advanced (3GPP LTE-Advanced), there is a need to develop solutions that can provide a better user experience while reducing the cost of infrastructure. Deployment of relay nodes is one such method wherein the base station (or eNB) communicates with a user equipment (UE) with the help of an intermediate relay node (RN), for example, when the distance between eNB and UE exceeds the radio transmission range of the nodes, or a physical barrier or radio frequency (RF) obstruction is present between the eNB and UE to degrade the channel quality. Generally, there can be more than one RN communicating data between an eNB and UE. In such situations, each intermediate node routes packets, e.g., data and control information, to the next node along the route until the packets reach their final destination.
Networks implementing single hop links between an eNB and a UE can severely stress link budgets at the cell boundaries and often render the users at the cell edge incapable of communicating or using higher data rates. Pockets of poor-coverage areas or coverage holes are created where communication becomes increasing difficult. This in turn brings down the overall system capacity as well as induces user service dissatisfaction. While such coverage voids can be avoided by deploying additional eNBs this significantly increases both the capital expenditure (CAPEX) and operational expenditure (OPEX) for the network deployment. A more cost effective solution is to deploy relay nodes (RNs) (also known as relays or repeaters) in areas with poor coverage and repeat transmissions so that subscribers in these coverage areas can be served better.
Even with the deployment of relays within a network, there remain some mechanisms that can further reduce costs. Typically, the RN not only provides an access link for delivering traffic to and from the UEs but it also routes this traffic (wirelessly) through the donor eNB and, hence, also supports a backhaul link. The RN thus uses the same resources (e.g., frequency, time, spatial, spreading codes, etc.) as a typical UE being served by the eNB. At the same time, the RN is expected to act as an infrastructure entity to serve another set of users (hereby referred to as UE2). A RN that shares the same resources as a UE is referred to an in-band RN while an out-of-band RN does not share resources with a UE.
In a relay node based upon the 3GPP Universal Mobile Telecommunications System (UMTS) LTE Release 8 (Rel-8) wireless communication system, the RN in time division duplex (TDD) mode is required to enable up to four links; two backhaul links (eNB to relay, relay to eNB) and two access links (relay to UE, UE to relay). To reduce complexity and interference, it is preferable for the relay to not be simultaneously (or concurrently) transmitting and receiving in the same frequency band. Thus, for instance, the relay cannot transmit to the eNB and receive from the UE concurrently and hence the eNB to relay and the relay to UE links must be time-multiplexed with a sufficient switching gap provided at the relay node. Furthermore, the relay design should be backward-compatible.
The RN can signal a Multimedia Broadcast multicast service Single Frequency Network (MBSFN) sub-frame in the downlink to a UE served by the RN to inform the UE that it does not receive downlink data transmissions, e.g., via the Physical Downlink Shared Channel (PDSCH), of the MBSFN sub-frame. The UE served by the RN also does not monitor reference symbols (e.g., for CQI or handoff measurements) outside of the control region of the MBSFN sub-frame transmitted to the UE. Thus, the RN can communicate with the eNB in the time-interval corresponding to the portion of the MBSFN when the UE is not expecting data from the RN. Furthermore, in a Time-Division Duplex (TDD) system, the relay and eNB may have different UL/DL configurations, wherein each configuration specifies a split of the resources (time resources) as downlink or uplink. Thus, there are several problems for designing the access/backhaul design depending on the configuration. For example, if there are no sub-frames that can be labeled as MBSFN for a configuration, then it may be difficult to design an eNB to RN link. Similarly, if both eNB and RN are in the uplink mode, then it is difficult to schedule the RN to do uplink transmission as it might lead to a loss of the acknowledgements from the UE transmitting to the relay. Furthermore, if two different configurations are used in the eNB (say uplink) and RN (downlink), then the RN DL control region can potentially interfere with uplink transmissions from UEs to the eNB. Therefore, there is a need to address the problem of interference as well as backhaul design for relay operating in TDD mode.
The various aspects, features and advantages of the invention will become more fully apparent to those having ordinary skill in the art upon a careful consideration of the following Detailed Description thereof with the accompanying drawings described below. The drawings may have been simplified for clarity and are not necessarily drawn to scale.